1. Field of the Invention
The present invention relates to a light-measuring device for measuring the intensity of light received using a means for detecting quantity of light, such as a photodiode.
2. Description of the Related Art
Conventionally, a light-measuring device such as an optical spectrum analyzer has been employed for the measurement of the amount of light of a light beam. FIG. 1 shows a block diagram of this type of light-measuring device of the prior art. This prior-art light-measuring device comprises a photodiode 2, a light detection circuit 23, an A/D converter 24, a computing unit 16, and a display 25.
Photodiode 2 receives the light to be measured and generates a signal that depends on the intensity of the light. Light detection circuit 23 amplifies the signal outputted by photodiode 2 at a fixed amplitude factor and outputs the result. A/D converter 24 performs A/D conversion of the signal outputted by light detection circuit 3 and outputs the result as measurement data. Computing unit 16 inputs the measurement data from A/D converter 24, carries out a process to convert the measurement data to data that can be displayed on display 25, and displays on display 25.
The light-measuring device of the prior art allows measurement of at various light intensities. The rated current that is allowed to flow through photodiode 2, however, is set, and flow of current greater than the rated current for more than a fixed time period results in breakdown of the photodiode.
In the light-measuring device of the prior art, therefore, the inadvertent incidence of light with intensity greater than the rated value for a period greater than the fixed period results in breakdown of photodiode 2.
As shown in FIG. 2, photodiode 2 has an output terminal connected to the grounded and an input terminal connected to the inverting input terminal of operational amplifier 3. The operational amplifier 3 has a non-inverting input terminal that is also grounded, and an output terminal that is fed back to be connected to the inverting input terminal via a feedback resistor 4.
In a light-measuring device of the above-described construction, photodiode 2 generates a signal depending on the amount of incident light when a light beam to be measured is supplied to photodiode 2. This signal is amplified and outputted by operational amplifier 3. This output allows measurement of the amount of light of the light beam.
However, the amount of light of the incident light beam varies from weak to strong, and as a result, if the gain of amplification by operational amplifier 3 is constant, then the range within which measurement can be effected will be limited.
To overcome this problem light-measuring device shown in FIG. 3 is known, in which feedback resistor 4 with variable resistance is provided, thereby expanding the range of measurement of the amount of light.
This light-measuring device of the prior art comprises an optical shutter 1, a photodiode 2, an amplifier 33, an A/D converter 24, a computing unit 16, a display 25, an operational amplifier 3, n resistors 8.sub.1 -8.sub.n with differing resistances, and a switch 10.
Switch 10 consists of n switches 9.sub.1 -9.sub.n, whereby a specific resistor among resistors 8.sub.1 -8.sub.n is connected to operational amplifier 3 by closing only the switch designated by gain switching signal 102.
Photodiode 2 converts the intensity of received light to a signal and outputs the result.
Operational amplifier 3 amplifies the signal from photodiode 2 based on the gain determined by the resistance of the resistor among resistors 8.sub.1 -8.sub.n that is connected and outputs the result.
Computing unit 16 both controls the turn on (light admitting)/turn off (light blocking) of optical shutter 1 based on optical shutter control signal 101, switches the gain of operational amplifier 3 based on gain switching signals 102 such that the measurement data outputted from A/D converter 24 fall within a certain fixed range, and causes the measurement data from A/D converter 24 to be displayed on display 25.
The operation of the light-measuring device of the prior art will be explained below. Computing unit 16 first turns on optical shutter 1 based on optical shutter control signal 101 and begins measurement of light intensity. The measurement data of the light intensity are then supplied to computing unit 16 by way of operational amplifier 3, amplifier 33, and A/D converter 24.
If the inputted data do not fall within a certain fixed range, computing unit 16 switches the gain of operational amplifier 3 by controlling switches 9.sub.1 -9.sub.n of switch 10 based on gain switching signals 102. At the time of switching this gain and for a fixed time interval afterwards, computing unit 16 turns off (light blocking) optical shutter 1 by means of optical shutter control signals 101 and measures the data delivered from A/D converter 24 during this time as the offset voltage of operational amplifier 3. The offset voltage is the voltage that appears in output at the operational amplifier when the input is zero.
Computing unit 16 then turns on optical shutter 1 based on optical shutter control signal 101, measures the light intensity, and subtracts the previously measured offset voltage from this measurement value and displays the resultant data on display 25 as the actual light intensity.
FIG. 4a shows a graph of the change in measurement data outputted from A/D converter 24 with respect to the measurement time, and FIG. 4b shows a graph of the change with respect to measurement time of data following subtraction of the offset voltage from the measurement data shown in FIG. 4a in computing unit 16.
In FIG. 4a, data measured during the time the optical shutter is turned off represent the offset voltage of operational amplifier 3, and the actual data as shown in FIG. 4b can be obtained by subtracting this offset voltage from the measurement data. The interval during which optical shutter is OFF is a display halt interval at display 25 because there are no measurement data to be displayed.
The amount of drift of an operational amplifier differs for each operational amplifier and in addition, varies with temperature. The offset voltage of the operational amplifier will therefore vary due to drift if there is a slight difference between the temperature of the operational amplifier at the time of measuring the offset voltage and the temperature of the operational amplifier at the time of measuring the actual light intensity, with the result that error is introduced in the measurement data.
Explanation will be next presented regarding measurement data in the case of drift of this offset voltage using FIG. 5a and FIG. 5b.
As shown in FIG. 5a, the amount of offset drifts during the interval that the optical shutter is turned off, and an average value is measured during this interval. The average value of this offset voltage is then subtracted from the measurement data of FIG. 5a to obtain the data shown in FIG. 5b. However, the obtained data inevitably includes error because the offset voltage drifts even after measuring the offset voltage.
Temperature drift in the offset voltage of the operational amplifier stops changing after a set time interval. An offset voltage value in which the effect of drift is eliminated can therefore be obtained if the offset voltage is measured after lengthening the time the shutter is turned off and drift stabilizes. Since measurement must be halted during the time interval, however, display on. display 25 must also be suspended.
In addition, the prior-art light-measuring device shown in FIG. 3 expands the measurement range of the amount of light because the gain of operational amplifier 3 can be changed. The drift of operational amplifier 3 also increases when the amount of light in a light beam with high intensity is measured because the output voltage increases and temperature rises, and measurement accuracy is therefore diminished if the amount of light in a light beam with low intensity is measured after measuring a light beam with high intensity.
Moreover, although the signal generated by the aforementioned photodiode is substantially directly proportional to the amount of incident light, some photodiodes exhibit a characteristic such that the generated signal is saturated when the photodiode is irradiated by a large amount of light. In a light-measuring device utilizing a photodiode with the saturation characteristic, the saturation characteristic can usually be improved by applying to the photodiode a bias voltage with opposite polarity to the generated current.
A light-measuring device that uses photodiode 12 having the aforementioned saturation characteristic will be next described with reference to FIG. 6.
In contrast to the light-measuring device of FIG. 2, this light-measuring device employs photodiode 12 having a saturation characteristic in place of photodiode 2, and in addition, is provided with bias power source 14 between the ground and output terminal of photodiode 12 for applying a reverse bias to photodiode 12 by way of resistor 13.
In the above-described light-measuring device, the application of a bias voltage with a polarity opposite to that of the generated current in photodiode 12 by bias power source 14 allows an improvement in the saturation characteristic of photodiode 12, which generates a current corresponding to the amount of light in a light beam.
In addition, the current generated by a photodiode not having this type of saturation characteristic is substantially directly proportional to the amount of light, and the photodiode is therefore energized to the output limit of operational amplifier 3 when the amount of light is excessive.
The photodiode is energized by an excessive current that surpasses the permissible rated current when the amount of light is excessive, and the photodiode is therefore subject to breakdown. When the current generated by photodiode 12 increases in the light-measuring device of FIG. 6, however, the voltage generated across resistor 13 also increases, whereby the bias voltage applied to photodiode 12 decreases and the saturation characteristic of photodiode 12, which had been suppressed, becomes conspicuous. The current flowing through photodiode 12 is then obstructed due to this saturation characteristic, with the result that the flow of excessive current through photodiode 12 can be suppressed.
The linearity of measurement of the amount of light decreases as this type of saturation characteristic becomes conspicuous, and the measurement accuracy of the amount of light therefore worsens as the amount of light increases.
In addition, when the light beam is of low intensity, the bias voltage of bias power source 14 gives rise to a dark current of photodiode 12, thereby creating an offset in the output result and degrading the measurement accuracy.
For this reason, in the light-measuring device shown in FIG. 7, relay 17 is provided between bias power source 14 and photodiode 12, so as to allow switching of the application of bias voltage.
Relay 17 allows the connection of the output terminal of photodiode 12 to either bias power source 14 or ground terminal 15.
Namely, in this light-measuring device, relay 17 allows connection of the output terminal of photodiode 12 is connected to ground terminal 15 when measuring a light beam with low intensity, thereby preventing the occurrence of offset voltage due to dark current in the output results.
However, relay 17 requires time for switching connections, and the above-described light-measuring device therefore encounters difficulties when continuously measuring light beams of from high intensity to low intensity. An analog switch can be considered for effecting this switching of connections, but an analog switch generates a large leakage current and therefore complicates accurate measurement when measuring the amount of light at low intensities.